The present invention relates to cementing operations and, more particularly, in certain embodiments, to methods and compositions that utilize pumice and various additives.
In cementing operations, such as well construction and remedial cementing, settable compositions are commonly utilized. As used herein, the term “settable composition” refers to a composition that hydraulically sets or otherwise develops compressive strength. Settable compositions may be used in primary cementing operations whereby pipe strings, such as casing and liners, are cemented in well bores. In a typical primary cementing operation, a settable composition may be pumped into an annulus between the exterior surface of the pipe string disposed therein and the walls of the well bore (or a larger conduit in the well bore). The settable composition may set in the annular space, thereby forming an annular sheath of hardened, substantially impermeable material (e.g., a cement sheath) that may support and position the pipe string in the well bore and may bond the exterior surface of the pipe string to the well bore walls (or the larger conduit). Among other things, the cement sheath surrounding the pipe string should function to prevent the migration of fluids in the annulus, as well as protecting the pipe string from corrosion. Settable compositions also may be used in remedial cementing methods, such as in squeeze cementing for sealing voids in a pipe string, cement sheath, gravel pack, subterranean formation, and the like.
In remedial cementing, settable compositions may be used for sealing voids in a pipe string or a cement sheath. As used herein the term “void” refers to any type of space, including fractures, holes, cracks, channels, spaces, and the like. Such voids may include: holes or cracks in the pipe strings; holes, cracks, spaces, or channels in the cement sheath; and very small spaces (commonly referred to as “microannuli”) between the cement sheath and the exterior surface of the well casing or formation. Sealing such voids may prevent the undesired flow of fluids (e.g., oil, gas, water, etc.) and/or fine solids into, or from, the well bore. The sealing of such voids, whether or not made deliberately, has been attempted by introducing a substance into the void and permitting it to remain therein to seal the void. If the substance does not fit into the void, a bridge, patch, or sheath may be formed over the void to possibly produce a termination of the undesired fluid flow. Substances used heretofore in methods to terminate the undesired passage of fluids through such voids include settable compositions comprising water and hydraulic cement, wherein the methods employ hydraulic pressure to force the settable composition into the void. Once placed into the void, the settable composition may be permitted to harden.
Remedial cementing operations also may be used to seal portions of subterranean formations or portions of gravel packs. The portions of the subterranean formation may include permeable portions of a formation and fractures (natural or otherwise) in the formation and other portions of the formation that may allow the undesired flow of fluid into, or from, the well bore. The portions of the gravel pack include those portions of the gravel pack, wherein it is desired to prevent the undesired flow of fluids into, or from, the well bore. A “gravel pack” is a term commonly used to refer to a volume of particulate materials (such as sand) placed into a well bore to at least partially reduce the migration of unconsolidated formation particulates into the well bore. While screenless gravel packing operations are becoming more common, gravel packing operations commonly involve placing a gravel pack screen in the well bore neighboring a desired portion of the subterranean formation, and packing the surrounding annulus between the screen and the well bore with particulate materials that are sized to prevent and inhibit the passage of formation solids through the gravel pack with produced fluids. Among other things, this method may allow sealing of the portion of the gravel pack to prevent the undesired flow of fluids without requiring the gravel pack's removal.
A broad variety of settable compositions have been used heretofore, including cement compositions comprising Portland cement. Portland cement is generally prepared from a mixture of raw materials comprising calcium oxide, silicon oxide, aluminum oxide, ferric oxide, and magnesium oxide. The mixture of the raw materials is heated in a kiln to approximately 2700° F., thereby initiating chemical reactions between the raw materials. In these reactions, crystalline compounds, dicalcium silicates, tricalcium silicates, tricalcium aluminates, and tetracalcium aluminoferrites, are formed. The product of these reactions is known as a clinker. The addition of a gypsum/anhydrate mixture to the clinker and the pulverization of the mixture results in a fine powder that will react to form a slurry upon the addition of water.
There are drawbacks, however, to the conventional preparation and use of Portland cement. The energy requirements to produce Portland cement are quite high, and heat loss during production can further cause actual energy requirements to be even greater. These factors contribute significantly to the relatively high cost of Portland cement. Generally, Portland cement is a major component of the cost of hydraulic cement compositions that comprise Portland cement. Recent Portland cement shortages, however, have further contributed to the rising cost of hydraulic cement compositions that comprise Portland cement.